The present invention relates to a crucible having a combustion chamber therearound for producing an adjustable thermal profile within the combustion chamber to control the crucible temperature.
It is known that the higher the amount of oxygen in an oxidant stream the higher the flame temperature resulting from combustion of the oxidant and fuel. Typically, pure oxygen utilized for stoichiometric combustion will have a higher flame temperature than stoichiometric combustion of the same fuel utilizing air as the oxidant. This situation can lead to localized over heating in the process and failure. The use of oxygen/fuel combustion is difficult when impinging on an alloy that has a melting point well below that of high temperature refractory. Even the use of air/fuel combustion has to be carefully monitored to prevent this localized over heating. It is further known that the combustion of fuel and air results in a high generation of combustion related pollutants than is generated with a similar amount of fuel and oxygen. Therefore it is desirable, when possible, to replace this air stream with oxygen to reduce this pollutant emission without overheating in the process. It is known that sub-stoichiometric or super-stoichiometric combustion occurs at significantly lower temperatures than combustion at stoichiometric rates. Therefore the primary fuel and oxidant are injected in a sub or super stoichiometric ratio, with the oxidant port first followed by the fuel port. When the oxidant is introduced first it creates an oxidizing boundary layer that the fuel injects through and is enclosed by inside the combustion chamber. This region of combustion, in the case of a sub-stoichiometric fuel/oxidant ratio, will generate a low flame temperature to protect the surrounding material. The gas generated from this sub-stoichiometric (excess fuel) combustion is a reform gas of hydrogen, carbon monoxide and minor amounts of methane, water and carbon dioxide. The temperature of the gas will be sufficient to burn readily with any additional oxidant added downstream. By adding the oxidant first and creating an oxidant boundary layer as in the case of 100 percent oxygen, there is no carbon build up due to cracked fuel as it all reacts to form reform gas. In the case of super stoichiometric combustion where there is an excess of oxidant, the resulting gas generated from the primary injection ports will be oxygen, water, carbon dioxide with minor amounts of other species. This gas will also be of sufficient temperature to react rapidly with any fuel injected downstream. By allowing controlled introduction of the remaining oxidant, with sub-stoichiometric combustion, or remaining fuel, with super-stoichiometric combustion, the thermal heat from combustion is allowed to release to the environment of the combustion chamber to prevent localized overheating. Additionally the control of the thermal heat release allows the flexibility to profile the heat release as desired.
Two prior U.S. patents to Dykema, No. 5,215,455 and No. 5,085,156, each teach a combustion process for nitrogen or for sulfur and nitrogen containing fuels where the fuel combustion is provided by staged oxygen in the form of air injected into two or more combustion regions. The first combustion region involves fuel rich stoichiometric conditions under which nitrogen chemically bound in the fuel is substantially converted to molecular nitrogen. The second and final combustion region has at least two stages in which the products from the combustion region are further combusted under a condition of fuel rich stoichiometry and the products from the first stage are combusted at an oxygen/fuel stoichiometric ratio. The prior Schirmer et al. patent, No. 4,927,349, is a method for burning nitrogen containing fuels in a two staged combustion having a rich-lean combustion process which includes introducing the fuel and at least one stream of primary air into the primary combustion region in a fuel air ratio above the stoichiometric ratio to establish a stabilized flame and maintaining the flame in the primary combustion region for a period of time and terminating the primary combustion while introducing at least one stream of secondary air into a secondary region. Two U.S. Patents to Khinkis, No. 5,013,236 and No. 5,158,445, each teach an ultra-low pollutant emission combustion process and apparatus for combusting fossil fuels in an elongated cyclonic primary combustion chamber having a first stage combustion and a secondary combustion chamber having a large amount of fuel with excess air. In the Gitman patent, No. 4,453,913, a recuperative burner has a central burner tube having a rich fuel-air ratio provided to a central burner tube and a lean fuel-air ratio provided to an outer burner tube. The Hagar patent, No. 4,801,261, is for an apparatus and method for delivery of combustion air in multiple zones and has combustion air fed to an ignition zone with other air feeds fed to the outer supplemental zone where the main combustion takes place.
This invention relates to a heated crucible having a combustion process and apparatus that allows the use of oxygen/fuel combustion for an adjustable thermal profile in a combustion chamber surround the crucible and which has low pollutant emissions and which prevents localized overheating in the combustion chamber and in the crucible. In many cases when air/fuel combustion is being utilized the air is pre-heated utilizing the waste flue gas exhaust from the combustion process. The invention can utilize pre-heated oxidant or fuel streams even in the case where 100 percent oxygen is utilized for the oxidant stream. As a result of the low temperature operation of the combustion system, there are no requirements for exotic high temperature materials for the crucible as is normally required when the oxidant stream is 100 percent oxygen.
The following definitions are provided in connection with the present technology and invention. The primary Injection Region is the point where the fuel and a portion of the oxidant is introduced into the combustion chamber. The combustion chamber is the primary chamber that the fuel and oxidant are introduced, mixed and burned. Sub-stoichiometric firing is when more fuel is present than can be reacted to completion by the available oxidant. Super stoichiometric is when more oxidant is present than can be reacted to completion by the available fuel. Reform gas is formed in the process of partial oxidation of a carbonaceous fuel with oxygen from water, carbon dioxide, molecular oxygen or another oxygen rich source to produce carbon monoxide and molecular hydrogen. This occurs without the production and deposition of atomic carbon. Oxygen enrichment is the addition of oxygen to air for the purpose of increasing the oxygen content and reducing the nitrogen content. Downstream is defined as the combustion products exhaust path towards the exit of the exhaust port and may be in a direct line or spaced around the chamber circumference. In the case of a single or multizone unit, each zone begins with a primary injection region and ends with stoichiometric combustion of that zone. Non-stoichiometric is defined as sub or super stoichiometric combustion. Oxidant is defined as that which contains oxygen in any proportion that supports combustion.